Connection Assembly For Drill Collars or Heavy Drill Pipes

ABSTRACT

The invention relates to a connection assembly ( 100 ) for drill collars or heavy drill pipes comprising: a male connection element which comprises, in its longitudinal direction, an at least partially threaded conical surface portion which tapers from the first portion towards its free end; a female connection element which comprises an at least partially threaded conical inner surface and a cylindrical inner surface ( 220 ), said two connection elements being complementary so that one can be assembled in the other by screwing, characterized in that the male connection element additionally comprises, between its free end and said conical surface portion, another surface portion whose conicity is less than that of said surface portion and which is dimensioned with respect to said cylindrical inner surface ( 220 ) such that there is a clearance by virtue of which these two surfaces bear one against the other when the assembly is subjected to a bending stress.

The present invention concerns connections for tubular or cylindrical elements and more particularly connections intended to form a string of elements for drilling wells, in particular oil wells. It concerns more precisely a connection assembly for drill collars and/or heavyweight drill pipes.

When drilling a well in the ground, it is necessary to connect portions of tubular conduits or other cylindrical elements to form a string of elements able to reach the required depth. In the field of drilling for oil, with the aim of extracting liquid or gaseous hydrocarbons, it is of fundamental importance that such connections guarantee in particular structural strength and pressure resistance where the elements are assembled together.

A drill string generally comprises four types of element, from top to bottom:

-   -   drill pipes,     -   heavyweight drill pipes,     -   drill collars,     -   drilling tool.

The pipes and collars have an axial bore running from end to end to provide a passage for fluids to be extracted or injected during drilling.

The pipes and collars are primarily distinguished by the fact that:

-   -   Individually, the drill pipes are the lightest of the three.         They generally consist of a welded assembly of a tube and two         tool joints. The drill pipes provide a link between the drilling         tool and the surface, are generally loaded in traction, and         serve to transmit rotation torque and drilling fluid to the         drilling tool.     -   Heavyweight drill pipes have a much larger cross section than         drill pipes and can be made in one piece; they are often loaded         in bending and have external wear pads on the body to reduce         contact with and therefore wear against the walls of the well.         They are useful as transition elements between drill collars and         drill pipes. They considerably reduce the vibrations generated         by the drilling tool when cutting rock.     -   Drill collars have very thick walls (of the order of several         inches, i.e. several tens of millimeters thickness); they are         often machined with a spiral on their exterior to provide a         passage for the drilling fluid between the exterior surface of         the drill collar and the wall of the well, to reduce         differential pressure sticking. They can also be externally         machined to produce slip and elevator recesses to facilitate         maneuvering them and using them. Their weight contributes to         exerting a cutting force on the drilling tool and to tensioning         the drill pipes.

The combination of the heavyweight drill pipes, drill collars and drilling tool is sometimes called the bottom hole assembly.

The invention is directed to connecting the lower elements of this kind of drill string.

The force of gravity acting on the heavyweight drill pipes and the drill collars in particular enables them to be used in compression in order to apply a penetration force to the cutting elements of the drilling tool at the bottom end of the drill string. The number of heavyweight drill pipes and drill collars used in a given string depends on the load that is to be applied at the drilling end. Moreover, the elements of the drill string are driven in rotation in order to impart a cutting motion to the drilling tool situated at its lower end.

The elements of the drill string and in particular the drill collars and the heavyweight drill pipes are therefore loaded both in compression and in torsion.

The pipes and collars constituting the drill string have connections at each of their two ends for assembling them together to obtain a required configuration.

This type of connection is usually obtained by means of complementary threaded surfaces disposed at the ends of the elements to be assembled, on respectively male and female portions, usually with seals clamped by two appropriate surfaces of said two portions. The threads are generally conical with a wide pitch (several millimeters) in order to facilitate fast assembly (a fraction of a turn is sufficient to achieve clamping).

Connections of this kind with a conical thread for oil drilling elements are standardized by the API (American Petroleum Institute) in the standard API Specification 7, taken here as a reference.

A connection as defined by that standard consists of a male element including a shoulder and a conical portion that is at least partially threaded and extends from this shoulder to the free end of the element, and a complementary female element having a first threaded conical interior surface with the same taper as the conical surface of the male element, but shorter than the latter, and extended by another, smooth interior conical surface with a smaller taper than said first conical surface.

A problem encountered in this technical field and in particular with this type of connection results from the fact that the elements of the drill string, in particular the drill collars and the heavyweight drill pipes, have radial dimensions several times less than their longitudinal dimensions (typically, for a heavyweight drill pipe: 900 cm length, 25 cm diameter). Moreover, the diameter of the boreholes in which heavyweight drill pipes and drill collars are used is much larger than their own diameter. The diameter of a borehole is generally 17½ inches (approximately 44.5 cm).

Consequently, being subjected to compression stresses, these strings of drilling elements are inherently stressed in such a way that they deform by buckling in the drilled hole. This buckling is locally reflected in bending forces, in particular at the connections between the elements constituting the string. In practice, these connections constitute geometrical discontinuities (in particular because of the reduction of cross section) compared to the remainder of the string; thus in the event of buckling of the string they are the site of stress concentrations, with the consequent high risk of failure at this level.

In particular, torsion forces combined with repetitive bending forces at the connections typically lead to problems of fatigue failures caused primarily by stress concentrations in the female element, in particular at the circle forming the limit between said two conical interior surfaces. It has been found that fatigue failures occur in practice on this circle, some portions whereof are loaded in compression, others in tension, depending on the random angular orientation of the buckling of the drill string.

The present invention aims to alleviate these drawbacks and consists in a connection assembly for drilling elements providing an optimum response to the API Specification 7 standard but able to resist high internal stresses resulting from torsion or compression forces or buckling of the drill string, and thereby to prevent failure of elements of the connection, in particular fatigue failures. It aims in particular to reduce the maximum value of the local stresses encountered at the connection.

To this end it proposes a connection assembly for drill collars or heavyweight drill pipes, comprising:

-   -   a male connecting element having a substantially circular         section about a longitudinal axis and that includes within its         longitudinal direction, respectively:         -   a substantially cylindrical first portion forming a shoulder             at one of its ends,         -   an at least partly threaded second conical surface portion             tapering from the first portion toward the free end of the             element,     -   a substantially cylindrical female connecting element,         complementary to said male connecting element, including:         -   a radial portion forming a bearing face for said shoulder of             the first element and, longitudinally, coaxially and             respectively, from that portion,         -   an at least partially threaded conical interior surface             complementary to the at least partially threaded conical             surface of the male element,         -   a cylindrical interior surface,             said two connecting elements being intended to be assembled             one into the other by a screwing action entailing several             turns of the threads, characterized in that the male             connecting element further includes, between said second             portion and its free end, a third surface portion of lower             taper than the second portion of the conical surface and             sized relative to said cylindrical interior surface so that             there exists a gap whereby these two surfaces come             respectively into bearing contact if the assembly is             stressed in bending.

Thus the present invention reduces the stresses encountered at the connections, resulting in particular from buckling of the elements, and thereby increases the resistance to fatigue failure at the connections.

The female element preferably has the standard dimensions defined by the API Specification 7 standard and the male element is preferably the only element of the connection assembly to be modified, although remaining compatible with standard female elements according to that standard.

Thus the invention further proposes a male connecting element for heavyweight drill pipes or drill collars intended to cooperate with a female element and having a substantially circular shape about a longitudinal axis, including in its longitudinal direction:

-   -   a substantially cylindrical first portion forming a shoulder at         one of its ends,     -   an at least partly threaded second conical surface portion         tapering from the first portion toward the free end of the         element,

characterized in that the male connecting element further includes, between said second portion and its free end, a third surface portion of smaller taper than the second conical surface portion.

The document FR-2 508 970 has already proposed using cylindrical guide bearing surfaces on each of the elements to be assembled, on either side of the threaded conical portions, said bearing surfaces having dimensions such that they are nested one within the other before the threads interengage, with the objective of ensuring correct alignment thereof prior to screwing, to prevent damaging the threads. However, the above document relates to a field very different to that of drilling for oil, since its field is that of production tubing, especially in an offshore environment.

The tubular elements described by the above document do not constitute drilling equipment as such. They are used in a quasi-static manner when a previously drilled hole is being exploited and are not subjected to torsion and compression forces as encountered in drilling equipment. These elements are subjected only to tension forces caused by their own weight. The function of the connections between these elements is limited to providing a seal between said elements and the external environment. The mechanical stresses applied to these tubing elements are therefore totally different from those of the present invention. Moreover, in the type of tubing described by the document FR 2 508 970, the main problem is guiding the thread when assembling two elements to avoid damaging it when screwing them together.

In contrast, the connections between drill pipes that are the subject matter of the present invention generally do not give rise to any screwing problem and do not need guide elements.

Now, the present invention aims to reduce, at the connections between drilling elements, the concentration of stresses generated by buckling caused by compression/torsion forces acting on the string.

It follows from the above remarks that the person skilled in the art seeking to improve connections for drilling elements has no a priori reason to call upon knowledge from the diametrically opposite technique of connections for production tubing, all the more so in that the standards applicable to drilling are very strict and in practice act as a disincentive to the person skilled in the art seeking to alter them.

The document U.S. Pat. No. 5,908,212 A discloses a tool joint with high strength in torsion, including two shoulders, entailing the coming into abutment of shoulders situated on respective opposite sides of the threaded area on the male portion and the female portion, which rules out any possibility of taking up the gap between the tip of the male element and the internal surface of the corresponding female element. Thus a tool joint of this kind is unable to solve the problem of reducing the maximum value of the local stresses encountered at the connection.

The same applies to the document WO 2005/095840 A1, which describes a tool joint with high strength in torque, with double shoulders, which also rules out any possibility of taking up the gap, for the same reasons.

According to advantageous features of the invention, which may where appropriate be combined:

-   -   the male element includes a groove disposed between said second         portion and said third portion so that, when said two elements         are assembled together, and when said shoulder is abutted         against the radial bearing face, the geometrical limit between         said conical interior surface and said cylindrical interior         surface is in the space formed by said groove;     -   the third surface portion of the male element forming a bearing         surface is substantially cylindrical;     -   said portion is partially conical at its free end;     -   said taper is less than that of the at least partly threaded         second conical portion;     -   the third portion has a longitudinal dimension at most equal to         ⅔ of the longitudinal dimension of the second portion;     -   the third portion has a longitudinal dimension from 10% to 50%         of its diameter;     -   the groove has a radius of curvature from ½ to 3 inches, i.e.         between approximately 12 millimeters and 76 millimeters,         depending on the diameter of the connections;     -   the two elements are axially bored and open-ended.

Thus the present invention proposes to divide the stresses to either side of the limit between the two surfaces, without stressing this critical area directly through mechanical contact.

In other words, for the same flexing of the connection, the invention proposes to divide the forces resulting from that flexing over a larger contact area between the two elements constituting the connection, and therefore to reduce the value of the maximum stresses encountered (the stress being by definition a force per unit area).

Moreover, adding a bearing surface with a low taper, or even a cylindrical bearing surface, beyond this critical area, displaces some of the forces to an area at a greater distance from the free end of the female element toward a portion of that female element where the walls have a larger cross section providing better resistance to mechanical stresses.

The features and advantages of the invention will emerge from the following description, given by way of nonlimiting illustration, with reference to the appended drawings in which:

FIG. 1 is a view in longitudinal section of a standard female element conforming to the API standard.

FIG. 2 is a view in longitudinal section of a male element according to the invention.

FIG. 3 is a view in longitudinal section of the aforementioned two elements when assembled in accordance with the invention.

As can be seen in FIG. 3, a connection assembly 100 according to the invention is made up of a male element 300 and a female element 200 assembled together. These two elements can be seen separately in FIGS. 2 and 1, respectively.

The male connecting element 300 has a circular section and includes in its longitudinal direction a substantially cylindrical first portion 302 conformed at one of its ends as a shoulder 301 and adapted to cooperate at its opposite end with a drill string element (not shown), a conical portion 310 provided with a thread 311, a groove 320 being provided between said substantially cylindrical portion 302 conformed as a shoulder 301 and the conical portion 310, and a substantially cylindrical portion 330 forming the end of the element 300 and connected to the conical portion 310 by a groove 340.

This groove advantageously has a radius of curvature from ½ to 3 inches according to the diameters of the connections, i.e. from approximately 12 millimeters to 76 millimeters.

The connecting element 300 has at the end intended to cooperate with a string element a substantially cylindrical bore 350 terminating in a conical portion 360 that widens to an outlet at the opposite end of the connecting element 300.

The female connecting element 200 is also open-ended and has a substantially cylindrical exterior shape and, at its end opposite the end intended to cooperate with a drill string element (not shown), a radial face 201 forming a bearing face to cooperate with said shoulder 301, and, in the longitudinal direction, respectively coaxially, a conical interior surface portion 210 partially provided with a thread 211 and complementary to the conical surface 310 provided with the thread 311, a substantially cylindrical interior surface portion 220 intended to cooperate with the cylindrical portion 330, preferably with a snug sliding fit, a conical interior surface portion 230 with the same taper as the portion 210, an interior surface portion 240 of substantially higher taper than the portion 210, and a cylindrical surface portion 250 open at the end intended to cooperate with a drill string element (not shown).

As can be seen in FIG. 3, when the two elements 200 and 300 are assembled together, the shoulder 301 is abutted against the radial surface 201, and the bearing surface 330 substantially facing the bearing surface 220, a small gap, exaggerated to clarify the figure, being visible between these two bearing surfaces. This gap is advantageously close to a locating gap fit, ISO designation H7g6, for example (according to NF R91-011).

The contact between the shoulder 301 and the radial surface 201 seals the assembly.

The groove 340 straddles the conical surface 210 and the cylindrical surface 220, substantially facing the circle 260 forming the limit between these two surfaces.

Accordingly, when the assembly 100 is loaded in bending, the two bearing surfaces 330 and 220 come into contact and then into bearing engagement, which distributes the forces, reduces the stresses in the female element, without the critical circle and its immediate vicinity being loaded by contact.

Note that the grooves 320 and 340 can be used to engage and disengage a cutting tool for machining the thread 311 on the portion 310 that lies between them. The thread 311 can be produced by any other method, however.

The two grooves can also serve as expansion grooves for pressurized fluids in order to contribute to sealing the assembly.

These two grooves can advantageously receive seals (not shown), for example O-rings, to improve the sealing of the connection.

Instead of or in addition to what is described above, a groove (not shown) can be machined in the bearing surface 330 to accommodate a seal, for example an O-ring, again with a view to improving the sealing of the assembly, in particular during eruption drilling, for example.

The bearing surface 330 can advantageously be non-cylindrical but slightly conical (with a taper less than that of the threaded portions), at least partially, at its free end, to facilitate linear contact with the bearing surface 220 in the event of bending of the assembly, and to offer improved stress distribution.

These elements advantageously conform to the API Specification 7 standard.

More precisely, they advantageously have the following dimensions, for example (for an API standard NC50 connection):

-   -   maximum inside diameter of conical portion 210: approximately         153 millimeters;     -   minimum inside diameter of conical portion 210: approximately         134 millimeters;     -   total length of conical portion 210: approximately 114         millimeters;     -   threaded length of conical portion 210: approximately 98         millimeters;     -   length of cylindrical portion 220: approximately 51 millimeters;     -   inside diameter of cylindrical portion 220: approximately 134         millimeters;     -   length of conical portion 230: approximately 51 millimeters;     -   taper of portion 240: approximately 30°;     -   inside diameter of cylindrical portion 250: approximately 76         millimeters;     -   inside diameter of groove 320: approximately 138 millimeters;     -   maximum outside diameter of conical portion 310: approximately         154 millimeters;     -   length between shoulder 301 and farthest end of groove 340:         approximately 114 millimeters;     -   minimum diameter of groove 340: approximately 128 millimeters;     -   maximum width of groove 340: approximately 20 millimeters;     -   minimum radius of curvature of groove 340:     -   inside diameter of portion 350: approximately 76 millimeters;     -   maximum inside diameter of conical portion 360: approximately 83         millimeters;     -   gap at diameter between portions 330 and 220: H7g6 fit).

A test has indicated an approximate value of 60% for the reduction of stresses obtained in this way at a distance of 10 cm from the sealing bearing surface, consisting of the contact between the shoulder 301 and the radial surface 201 (this test was carried out with strain gauges placed on the exterior surface of the female element at distances of 2, 4, 6, 8 inches, i.e. 50.8, 101.6, 152.4, 203.2 millimeters, from the sealing bearing surface 201).

Note that many modifications or variations of the connections assembly described and shown can easily be made by the person skilled in the art without departing from the scope of the invention. 

1. Connection assembly (100) for drill collars or heavyweight drill pipes, comprising: a male connecting element (300) having a substantially circular section about a longitudinal axis and that includes within its longitudinal direction, respectively: a substantially cylindrical first portion (302) forming a shoulder (301) at one of its ends, an at least partly threaded second conical surface portion (310) tapering from the first portion toward the free end of the element, a substantially cylindrical female connecting element (200), complementary to said male connecting element, including: a radial portion (201) forming a bearing face for said shoulder (301) of the first element and, longitudinally, coaxially and respectively, from that portion, an at least partially threaded conical interior surface complementary to the at least partially threaded conical surface of the male element, a cylindrical interior surface, said two connecting elements being intended to be assembled one into the other by a screwing action entailing several turns of the threads, characterized in that the male connecting element further includes, between said second portion and its free end, a third surface portion of lower taper than the second portion of the conical surface and sized relative to said cylindrical interior surface so that there exists a gap whereby these two surfaces come respectively into bearing contact if the assembly is stressed in bending.
 2. Connection assembly (100) according to claim 1, characterized in that the male element (300) includes a groove (340) disposed between said second portion (310) and said third portion (330), so that, when said two elements are assembled together, and when said shoulder (301) is abutted against the radial bearing face, the geometrical limit between said conical interior surface and said cylindrical interior surface is in the space formed by said groove.
 3. Connection assembly (100) according to claim 1, characterized in that the third surface portion (330) of the male element forming a bearing surface is substantially cylindrical.
 4. Connection assembly (100) according to claim 3, characterized in that said portion (330) is partially conical at its free end.
 5. Connection assembly (100) according to claim 4, characterized in that said taper is less than that of the at least partly threaded second conical portion (310).
 6. Connection assembly (100) according to claim 1, characterized in that the third portion (330) has a longitudinal dimension at most equal to ⅔ of the longitudinal dimension of the second portion (310).
 7. Connection assembly (100) according to claim 1, characterized in that the third portion (330) has a longitudinal dimension from 10% to 50% of its diameter.
 8. Connection assembly (100) according to claim 1, characterized in that the groove (340) has a radius of curvature from ½ to 3 inches, i.e. from approximately 12 millimeters to 76 millimeters.
 9. Connection assembly (100) according to claim 1, characterized in that the two elements (200, 300) are axially bored and open-ended.
 10. Male connecting element (300) for heavyweight drill pipes or drill collars intended to cooperate with a female element (200) and having a substantially circular shape about a longitudinal axis, including in its longitudinal direction, respectively: a substantially cylindrical first portion (302) forming a shoulder (301) at one of its ends, an at least partly threaded second conical surface portion (310) tapering from the first portion toward the free end of the element, characterized in that the male connecting element further includes, between said second portion and its free end, a third surface portion (330) of smaller taper than the second conical surface portion. 